promiscuous sex outside the context of any relationship has little to do with relationship styles at all.

I'm not sure that's fair. Couldn't you say that having multiple friends with benefits is a type of " relationship style," in a sense?

monogamous norms are weakening, and these examples are the result of particular groups not having any strong relationship norms at all; various kinds of polyamory and swinging offer social norms that can be as strong as monogamous ones

Again, "take some time between serious relationships to have multiple casual partners" can be a relationship norm, one that appears very common.